A gas generator for rapidly inflating an air bag so as to protect a driver and etc, from an impact caused in a collision of the automobile, is incorporated into an air bag module mounted in a steering wheel. This gas generator instantaneously generates a large amount of high-temperature gas in response to a collision detection signal of a collision sensor in the event of collision.
As one example of the gas generator for inflating the air bag, there is one shown in FIG. 25. This gas generator includes a housing 100, formed by an upper case 101 and a lower case 102 of a double cylindrical structure of having lids. The housing 100 has a structure in which an annular airtight space S is formed in the inside, by abutting and friction-welding at inner cylindrical portions of the upper case 101 and a lower case 102 and outer cylindrical portions thereof. In the airtight space S in the housing 100, gas generating agents 103 and a cylindrical filter member 104 are successively housed from the inner cylindrical portion toward the outer cylindrical portion. In the inner cylindrical portion, an igniter 105 ignited in response to a collision detection signal from a collision sensor and an inflammation agent 106 fired by ignition of the igniter 105 are disposed.
In the gas generator, the igniter 105 is ignited in response to the collision detection signal from the collision sensor to fire the inflammation agent 106. A flame of the inflammation agent 106 spouts through an igniter hole 107 of the inner cylindrical portion into the airtight space S, and the gas generating agents 103 fires and burns, so that a large amount of high-temperature gas is instantaneously generated. The large amount of high-temperature gas flows into the filter member 104, is subjected to slag collection process and cooling process, and flows into an air bag from a plurality of gas discharge ports 101a of the upper case 101 from now on. The air bag is rapidly inflated by the large amount of clean gas discharged from the respective gas discharge ports 101a. 
In the prior-art gas generator, in the case the igniter ignites in response to the collision detection signal from the collision sensor, the large amount of clean gas is instantaneously generated, and then the air bag is rapidly inflated regardless of a form of the collision of the automobile or of a seated posture of the driver. Therefore, in the case the driver is seated near a steering wheel, or even the case the automobile collides at a low speed, the driver receives an impact from the air bag inflating rapidly. As a result, there is a problem that the air bag cannot show a inherent function of protecting the driver.
Therefore, it is an object of the present invention to provide a gas generator for allowing the inherent function of the air bag to be performed by slowly inflating the air bag in an initial stage of inflation and then rapidly inflating the air bag, or evenly discharge the clean gas around the housing.